Electro-optical system



Oct. 2 1 7 K; D; sMm-I 2,810,863

ElEC'I'RO-OPTICAL SYSTEM Filed July 1'7, 1945 2 Sheets-Sheet 1 FIG. I l5 1.9 h H 2 T w 216 10 ll 5:

& i514 z? 2 13 34 AIILZ IIIHV AAAAAAAAAA vvvvvvv uv VEN TOR K. 0. SMITH BV yaw? fl KM ATTORNEY Oct. 22, 1957 K. D. sMm-n ELECTRO-OPTICAL SYSTEM Filed July 17, 1943 2 Sheets-Sheet 2 INVEN 709 K 0. SMITH A TTORNEY 2,810,863 ELEC'I'RG-QPTICAL SYSTEM Kenneth 1 Smith, White Plains, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Appiication July 17, 1943, Serial No. 495,121 1% Ciairus. (Cl. 315159) This invention relates to radiant energy sensitive systems and more particularly to electro-optical systems.

An object of the invention is to provide an improved electro-optical system having a nearly constant percentage sensitivity, that is, a system in which the output voltage is nearly proportional to the percentage modulation of a modulated static condition but is independent of the magnitude of the static condition.

In order to attain this object it is proposed to utilize one or more non-linear resistive elements. A circuit typical of this invention comprises a photoelectric cell, a source of direct current potential and a varistor connected in series to which is coupled a vacuum tube amplifier through a series condenser. By using a properly proportioned varistor or combination of varistors and fixed resistances a nearly uniform sensitivity for a given percentage change in illumination over an extensive range of illuminations may be obtained.

Another object is to provide an improved means in a radiant energy sensitive system for controlling the time of response to the received energy.

A varistor, as the term is used herein, is a non-linear resistance device which may assume a variety of forms but is always a conductive or semiconductive device, never an electron emissive device. Varistor elements suitablefor use in embodiments of this invention are discs of a semiceramic material composed of silicon carbide, clay and carbon, pressed and fired. The impedance at any given voltage is inversely proportional to the area of the disc. Therefore almost any desired operating impedance may be realized by a choice of area of the varistor disc. This type of varistor, its composition and method of manufacture, are described in McEachron Patent 1,822,742, issued September 8, 1931.

One use to which this invention can be put is to detonate the explosive charge of a projectile as it moves into proximity to a target due to a reduction in the illumination of the photoelectric cell. The normal or static illumination may be large or small but for any given percentage reduction in the illumination the sensitivity remains practically constant. Thus, as the projectile moves past the target, which may be an airplane, a certain percentage of the illumination of the photoelectric cell is cut oif at a given rate, dependent upon the speed of the projectile and the nature of the airplane. However, the sensitivity of the detonating unit is reasonably constant no matter what the absolute intensity of the illumination may have been.

In one circuit arrangement devised for such a use a single-stage vacuum tube amplifier is inserted between the photoelectric cell and varistor circuit and a thyratron stage which on firing causes detonation of the explosive charge. In this arrangement the input circuit of the amplifier is connected across the varistor so that a reduction in illumination of the photoelectric cell tends to make the grid of the thyratron more positive with respect to its cathode.

In another circuit arrangement also devised for such use a two-stage amplifier is inserted between the photoelectric cell and varistor circuit and the thyratron stage. In this second arrangement the input circuit of the first amplifier stage is connected across the photoelectric cell.

rates Patent a ice 2 This connection to the photoelectric cell causes the grid of the thyratron also to become more positive when the illumination on the photoelectric cell is reduced.

These circuit arrangements consist basically of three parts: (1) the circuit for converting light fluctuations into voltage changes closely proportional to the percentage light fluctuations; (2) a selective amplifier to amplify corresponding voltage fluctuations to a suitable intensity; and (3) a trigger device to supply, when acted upon by the desired signal, sufiicient direct current to detonate the explosive charge. Now if we designate the peak pulse voltage necessary to fire the trigger tube by V, the voltage gain ratio of the amplifier by G, and the conversion factor of the input circuit, that is, volts output per percent light fluctuations, by E, then we may indicate the sensitivity of the circuit in percentage change of illumination required to fire the thyratron by the following equation:

V Sens1t1v1ty E G (1) To obtain increased sensitivity that is, a lowered percentage change of illumination required to fire the thyratron, we may operate on any or all of the factors V, E and G which are subject to design variations. If we wish to use fewer amplifying tubes, resulting in a lower gain G, we may at least partially compensate for such reduction by reducing the necessary peak pulse voltage V and increasing the conversion factor E.

The peak pulse voltage required is dependent upon the negative bias used on the thyratron. It can be seen from Equation 1 that if V and G are changed in the same ratio the sensitivity is not efiected so that if G is reduced, say to one-half its previous value, the lost sensitivity may be regained by reducing the negative bias, therefore permitting reducing V to one-half its previous value. This can be done since the noise disturbances and the signal are amplified together and such modification does not change the signal-to-noise ratio nor make the circuit more susceptible to firing on noise. The amount of allowable reduction of negative bias is dependent upon variations of bias voltage from battery to battery. Consequently, in any practical circuit there is a limit to which the peak pulse voltage V may be reduced.

If we make the assumption that all significant disturbances are present in the amplifier input, then changing the amplifier gain G only changes the sensitivity of the unit. If we economize on circuit elements by leaving out a tube and its associated coupling impedances we will of course broaden the frequency responses but this can be tolerated at the reduced sensitivity.

Using a varistor in combination with a photoelectric cell in accordance with this invention we have almost complete control of the general impedance level of the load circuit. We can make it equal to or greater than that of the photoelectric cell at the usual light intensities and thus obtain a conversion factor which is relatively high. An advantage of this high conversion factor is that it is practicable with one amplifier tube to operate at a sensitivity of about 1 percent. Furthermore, with the better conversion factor and reduced gain, noise and vibration disturbances in the first stage are not so troublesome, resulting in a saving in vacuum tube cost.

The invention will now be described more in detail, having reference to the accompanying drawings.

Fig. 1 is a circuit diagram showing one embodiment of the invention having a single amplifier stage;

Fig. 2 is a circuit diagram of a modified embodiment of the invention;

Fig. 3 is a circuit diagram of another modified embodiment of the invention having a two-stage amplifier;

Figs. 4 and 5 illustrate other embodiments of the invention having a plurality of varistors and fixed resistors.

'The same reference characters are used to indicate identical elements in the several'figuresof the drawings. An embodiment of the invention having a single amphfi e r stage will now be described having reference to Fig.1.

A photoelectric cell IiL-a-varistor 11, a battery 1-2 and a -protective resistor 13 are connectedinseties?",Coupledj across the varistor 11 i's'anamplifier staged/I too-which amplifier stage iscoupled a thyratron-stage The amplifier stage VI- comprises a vacuum -tube 14,-the control, grid of which is connectedthrough a .c'ondenser fli to that terminal er vari'stor 11- which "is qeneeeted to the cathode of photoelectric cell 10. "Thelcathode of v a c'uum tube 14 is connected to the other terminalof varistor 11f Theinput circuit of a'r'nplifier stage VI is completed by grid leak resistor 16 which is connected between the grid and cathode of vacuum tube 14.; The output circuit'of vacuum tube '14 is coupled tothe input circuit of thyratron 18 by resistor 17, series condenser 19 and a' grid leak combination .of resistors 20, 21 and 22. A negative bias for the grid of thyratron 18 is suppliedby battery 23. "Resistors 21 and 22 constitute'a voltage divider connected across battery 23 whereby a desired negative bias forthe grid of thyratron'18 is obtained. The resistor 25 serves as load for thyratron stage T and in series with battery 1 2'is connected between the anode and cathode of thyratron 18. The voltage for the screen grid of vacuum tube 14 is supplied from an intermediate tap 26 on battery; 12. Thenegative terminalof battery 12 is grounded by conductor 24.

; By reason of the voltage versus current characteristic of varistor 11 approximately equal voltage changes across the terminals of varistor 11 occur for equal percentage changes of illumination on photoelectric cell 10, regardless of the absolute illumination of photoelectric cell within the operable range of illuminations. However, an

impulsive reduction of light on photoelectric cell 10 causes an impulsive change of voltage on the grid ofvacuum tube 14 which is transmitted to the grid of thyratron 18, causing it to fire and energize load resistor 25, Resistor 13 protects the photoelectric cell10iagainst excessive currents on high illuminations which would otherwise result because of the low resistance of varistor 11 at high current values. r v

The modified circuit of Fig. 2 is in many respects identical with the circuit of Fig. 1. A photoelectric cell 10,

a varistor 11 and a battery 12 areadaptedtobe connected in a closed series circuit upon the closure of switch 27. Coupled across the varistor 11 is an amplifier stageVi to whichis coupled thyratron stage. T. The amplifier stage VI comprises a vacuum tube 14, the control grid of which is connected through a condenser to that terminal of varistor 11 which is connected to the cathode j of photoelectric cell 10. The cathode, of vacuum. tube 14 is connected to the other terminal of varistor 11. The

input circuit of amplifier stage VI is completed bygrid leak resistor 16 which is connected between the grid and cathode of vacuum tube 14. The output circuitcf vacuum tube 14; is coupled to the input circuit of thyratron. 18 by resistor ij series condenser 19, shunt condenser.

28, a series arrangement of grid'leak resistors 29 and 3t; and negative biasing battery 31., The resistor serves as load for thyratron stage -T and is connected upon closure of switch 32 in series with battery 12 and switch 27 between the cathode and anode of thyratron 18. .Bat-

tery '12 is shunted by a condenser 33. When thyratron 1'8 fire s, resistor 25 is heated sutficiently to cause detonation of the explosive charge of the projectile which, in a specific embodiment of the invention, is .controllediby the arrangement-illustrated in Fig. 2.

In the specific embodiment just mentioned, the ,thyratron 18 is adapted'to be fired after a predetermined elapsed m f w n the chains o swi s i 'ir rov d nst t has not been fired previouslypnder control of photoelectric cell 10. Such firing of thyratron. 18 is effected ye er i s of condenser 35 ih qi hi esi t fifi an i 37 in series from battery 4 12 after switch 27 has been closed. When condenser 35 acquires a predetermined charge, gaseous discharge lamp 38 which maybe a neon tube, breaks down placing a sufiicient positive voltage on discharge pathbetween cathode, and anode of thyratron nected across condenser 33 to prevent charging of condenser. 33 by leakage current across the contacts of switch 27 in its open condition and the consequent'charg ing of condenser 35 to the breakdown potential of discharge lamp 38. The resistance of resistor 34 is so-high' that the current drain on battery 12 is negligible" even 'when switch 27 is closed.

The screen gridi of vacuum tube 14 is connected through resistor 86 to the'anode of thyratron 18. Grid 85, therefore, is not energized .until switch 32 is closed.

Furthermore, condenser 87 j which .is connected between 7 grid 85 and the negative or zero terminal of-battery 12 must be charged through resistor 86 and load resistor 25 before the grid 85 assumes its operating potential. 'AS:

a result of this arrangement thyratron 18'cannotbe fired upon the. closing of switch 32 due to parasitic currents in the input circuit of amplifier stageVI because vacuum tube 14 is blocked when there is no positive voltage on grid 85, as at the instant that switch32-is closed and for a, fraction of a second thereafter or until condenser 87 i can be charged by current from battery 12 flowing through resistors 25 and 86.

, In the operation of the specific circuit of Fig. 2 when used to detonate the explosive charge of aprojectile, switches 27 and 32 are closed under the controlof forces vof acceleration resulting from the'translatory motion of the projectile. Switch 27 is closed first and starts the charging of condenser 35. Shortly thereafter switch-32 J is closed, placing positive'battery on the anode of thyratron 18 and activating amplifier stage VI by charging condenser 87 and energizing. screens grid85; of vacuum tube14. From theinstanti that amplifier stage VI is activated until the breakdown of discharge; device. 38, the

firing of thyratron stage T is under control of photoelece tric cell 10. This time interval is so chosen that the projectile during that interval is passing through the target zone. Ifit passes in proximity to a target animpulsive reduction: in illumination of the' photoelectric cell 10 occurs causing the thyratron. 18 to fire. Since the re sponse is approximately constant for any given percentage" -reduction of illumination, within the operatingrange of illuminations, thiscircuit is efiectiveto-firethe thyratron" 18 whether'the illumination be great or W small. important since the illumination from the sky in this specific embodiment.

jFOI testing. purposes manually operated switch88 is provided which when closed prevents the charging of condenser 35 to thebreakdown potential of discharge tube 38. i l 12 .while switch 88 is closed.

'The cathodes of vacuum tube 14and thyratron 18 are energized from battery 55.

The arrangement of Fig. 3 is somewhat like that of Fig. 1. Two stages of amplification are used between .the photoelectric cell and the varistor circuit and the thyratron stage T. The photoelectric cell 10, van'stor 11,

a protective resistor 13 and the battery 12 are connectedphotoelectric cell in a series circuit. Coupled across the 10is an amplifier having two stages V1, and V2to the output of which stage V2 is coupled thyratroni stagejTw -The amplifier stage V1 comprises a vacuum tube ,14', the control grid of Which is connected. through condenser' 15 to theanode of photoelectric cell10l is conv V This is i illumination relied upon is natural Resistor 37 prevents undue drain on battery nected to one terminal of varistor 11. The cathode of vacuum tube 14 is connected to the cathode of photoelectric cell lt). The input circuit of amplifier stage V1 is completed by grid leak resistor 16. The output circuit of amplifier stage V1 is coupled to the input circuit of amplifier stage V2 by resistor 17, series condenser 19, series grid resistor 41, grid leak resistors 42 and 43, shunting condenser 44 for resistor 43 and shunt grid condenser 45. The output circuit of amplifier stage V2 is coupled to the input circuit of thyratron stage T by resistors 46 and 47, condenser 48 shunting resistor 47 and battery 12, series condenser 49, grid leak resistor 52 and negative grid biasing battery 23. The output circuit of thyratron stage T comprises load resistor 25 in series with protective resistor 53 and battery 12. Resistor 53 may be short-circuited by the closure of switch 54. With resistor 53 in circuit the anode current of thyratron 18 cannot exceed a very small value, which will not appreciably afiect load resistor 25. Therefore resistor 53 serves as a protective resistance until switch 54 is closed to efiectively remove resistor 53 from the circuit. Switch 54 may be controlled manually or automatically as by forces of acceleration in the case where this arrangement is used to detonate an explosive projectile. The cathodes of vacuum tubes 14 and 4t} and thyratron 18 are heated by current from battery 55. The screen grids of vacuum tubes 14 and 49 are energized by a connection from intermediate tap 26 on battery 12 through resistors 50 and 51 respectively.

Vacuum tubes 14 and 40 of Figs. 1, 2 and 3 may be pentode tubes having suppressor grids which are connected to the cathodes.

A reduction in illumination of photoelectric cell in Fig. 3 causes a positive impulse of voltage to be applied through condenser 49 to the grid of thyratron 18 by reason of the voltage across photoelectric cell 10 being applied to the input circuit of amplifier stage V1 and two stages of amplification being used. Varistor 11 causes changes in this voltage to be of approximately equal amounts for any given percentage change of illumination of the photoelectric cell whether the initial illumination just prior to each change be large or small if it is Within the operable range of illuminations.

In the arrangement of Fig. 4 two varistors 6i and 62 are connected in series. Fixed resistor 61 is connected in series with varistor 6d, the input circuit of amplifier stage V1 including series condenser and grid leak resistor 16 being connected across both the varistor 69 and the resistor 61. A resistor 63 is connected in shunt of varistor 62. The primary series circuit, therefore, comprises photoelectric cell li), resistor 61, varistor 69, varistor 62 shunted by resistor 63 and battery 64. Suitable values for the fixed resistors are resistor 61, 2 megohms; resistor 63, 4 megohms; and resistor 16, 100 megohms. Any suitable circuit arrangement may be connected to the cathode and anode of tube 14 such as those illustrated in Figs. 1 and 2.

In the arrangement of Fig. 5, two varistors 76 and 62 are connected in series. The input circuit of amplifier V1, including condenser 15 and resistor 16, is connected across varistor 70. A protective resistor 71 is included in the primary series circuit which includes photoelectric cell 10, varistor 76, varistor 62 shunted by resistor 63, battery 64 and protective resistor 71. Suitable values for the fixed resistors 63 and 16 are the same as those given for the same resistors of Fig. 4 and for resistor 71, 3 megohms. As in the case in Fig. 4, any suitable circuit may be connected to the cathode and anode of tube 14 in Fig. 5.

The circuits of Figs. 1, 3 and 5 with their protective resistors 13 and 71 respectively, are particularly useful if the photoelectric cell 10 should become so oriented in flight that it would receive direct illumination from the sun, causing a tremendous pulse of light. The large value of the protective resistances at large light values 65 causes most of the battery voltage to appear across this resistance, leaving only a small voltage to cause current to flow through the photoelectric cell.

Other forms of input circuits may comprise a combination of varistor with positive input tube grid, a combination of varistor and variable negative grid bias, and a combination of varistor and cathode impedance.

What is claimed is:

1. An electro-optical system comprising a primary series circuit including a light sensitive device and a resistor, the resistance of said resistor changing in response to changes of illumination of said device, a vacuum tube amplifier having an input circuit including a series condenser and an output circuit, said input circuit being coupled to a portion of said series circuit across which there is a voitage drop dependent upon the illumination of the light sensitive device, and means connected to the output circuit of said amplifier adapted to be controlled by impulsive changes of light on said light sensitive device.

2. An electro-optical system comprising a primary series circuit including a light sensitive device and a resistor, the resistance of said resistor changing in response to changes of illumination of said device, a vacuum tube amplifier having an input circuit including a series condenser and an output circuit, said input circuit being connected across said resistor, and means connected to the output ircuit of said amplifier adapted to be controlled by impulsive changes of light on said light sensitive device.

3. An electro-optical system comprising a primary series circuit including a light sensitive device, a resistor of fixed resistance and a second resistor, the resistance of said second resistor changing in response to changes of illumination of said device, a vacuum tube amplifier having an input circuit including a series condenser and an output circuit, said input circuit being coupled to a portion of said series circuit across which there is a voltage drop dependent upon the illumination of said light sensitive device, and means connected to the output circuit of said amplifier adapted to be controlled by impulsive changes of light on said light sensitive device.

4. An electro-optical system comprising a primary series circuit including a light sensitive device, a resistor of fixed resistance and a second resistor, the resistance of said second resistor changing in response to changes of illumination of said device, a vacuum tube amplifier having an input circuit including a series condenser and an output circuit, said input circuit being connected across said second resistor, and means connected to the output circuit of said amplifier adapted to be controlled by impulsive changes of light on said light sensitive device.

5. An electro-optical system comprising a primary series circuit including a light sensitive device, a resistor of fixed resistance and a second resistor, the resistance of said second resistor changing in response to changes of illumination of said device, a vacuum tube amplifier having an input circuit including a series condenser and an output circuit, said input circuit being connected across both said resistor of fixed resistance and said second resistor, and means connected to the output circuit of said amplifier adapted to be controlled by the impulsive changes of light on said light sensitive device.

6. An electro-optical system comprising a series circuit including a photoelectric cell, a first varistor, a second varistor, a fixed resistor and a source of unidirectional current, and a vacuum tube amplifier having an input circuit including a condenser and an output circuit, said input circuit being connected across a portion of said series circuit including said first varistor.

7. An electro-optical system comprising a series circuit including a photoelectric cell, a first varistor, a second varistor, a fixed resistor and a source of unidirectional current, one terminal of said fixed resistor being connected tonne-terminal of said-first varistor, and a vacuum tube amplifier having an input circuit including a. condenser and an output circuit, said input circuit beingiconnected across a portion of said series circuit including both said first varistor and said fixed resistor. l

a 8. An electro-optical system comprising a series circuit including a photoelectric cell, a first varistor, a second varistor, a fixed resistor and a source of unidirectional current, a vacuum tube amplifier having an input circuit including a condenser and an output circuit, said input circuit being connected across a portion of said series circuit. including said first. varistor, and. a second fixed resistor connected in shunt to said second varistor.

'9. An electro-optical system comprising aseries circuitv including a photoelectric cell, a first varistor, a second varistor, a'fixed resistor and a source of unidirectional current, one terminal of 'saidfixed resistor being connected to one terminal of said first varistor, a vacuum tube amplifier having an input circuit including a condenser and an output circuit, said input circuit being connected across:

a portion of said series circuit including both said first varistor and said fixed resistor, and a second fixed resistor connectedin shunt to said second varistor.

'10. An electro-optical system comprising a series circuit including a photoelectric cell, a first varistor, a second varistor, a fixed resistor and a source of unidirectional current, and a vacuum tube amplifier having an input circuit including a condenser and'an output circuit, saidinput circuit being connected across said first varistor only. a

11. An electro-optical system comprising a series circuit including a photoelectric cell, a first varistor, a second varistor, a fixed resistor and a source of unidirectional current, a vacuum tube amplifier having an. input circuit including a condenser andan output circuit, said input circuit being connected across said first varistor only, and a second fixed resistor connected in shunt to'said second varistor;

121'An electro-optical system comprising a seriescircuit including a photoelectric cell, a varistor and a source of unidirectional current, a space discharge tube having.

a cathode, an anode and acontrol electrode, a series condenser connected between said control electrode and one terminal of said varistor, a conductive connection between'said cathode and the other'terminal of said varistor,

a fixed resistor connected between said control'electrode,

and said cathode, and means connected betweensaid anode and said cathode controlled by changes in-the illumination of said cell. 7

13. An electro-optical system comprising a series circuit includinga photoelectric cell, a varistor and a source of unidirectional current, a space discharge tube havinga cathode, an anode and a control electrode, a series condenser connected between said control electrode and one terminal of said varistor, a conductive connection between said cathode and the other terminal of said varistor, a fixed resistor connected between saidcontrol electrodeand said cathode, said fixed resistor having aresistance at least as large asthe maximum resistance of said varistor, and

means connected between said anode and said cathode controlled by changes in the illumination of said cell;

14. The combination with a vacuum'tube amplifierineluding a vacuum tube having a cathode,.. an anode, a

- first control electrode and a second control electrode of a radiant energy sensitive device, means including said radiant energy sensitive device for impressing signal voltage on said cathode and said first control electrode, means including a load device connectedto said cathode and anode, said means including switching means to operably associate said load device with said amplifier, and means for slowly applying an operating potential to said second control electrode only aftersaid switchingmeans is actuated to operably aSsociateTsaid load device. with said amplifier. i Q 7 i 15.;A radiant energy sensitive unit comprising a radiant energy sensitive device means for producing voltages under, controlof said device, a vacuumtube amplifier for amplifyingvoltages produced by said rne ans,. saidvacuum tube amplifier including ;a vacuum tube having a cathode, ananode, a control grid and a screen, grid,

afthyratroncoupled :to the cathode and anode of said; vacuum tube, said thyra tron having a cathode, an anode and a. control el ectrode, a load device, a source of unidirectional current; a switch, a, series circuit including said loaddevice, source of current and switch connected between the anode and cathode of said thyratron, a con denser connected between the screen; grid and cathode of the vacuum tube of said amplifier, and a high resistance connected between said screen grid and a point of said series circuit which is more positive than the cathodeof said thyratron and between; said switch and said anode.

16. An electronic 'circuit comprising a space, discharge tube having a cathode, an anode and a control electrode; means to apply control voltages between said' cathode and control electrode, a 'load device for said space discharge tube, a source of unidirectional current,-a switch, a series circuit including said load device, switch and source connected between said anode and cathode, a condenser connected in shunt of said source and switch, and a high resistance connected in ashunt circuit for said condenser to drain ofi any charge on said condenser due to leakage current from said source across the terminalsof said switchin its open position.

17. A space discharge tube amplifier comprising a space discharge tube having a cathode, am anodeand a control electrode, means to apply voltages to be amp'lifiedbetweengsaid cathode and control electrode, aload device for said space'discharge tube, a source-ofunidirectional current,a switch, a series circuit including said load devicekswitch and sourcesconnected between; said across the terminals of said switchinitsopen position,

and a gaseous discharge tube connected in a circuit from the positive terminal of said condenser to said" control electrode. Y 1 i e 1 l H 18. An electro-optical system comprising a. photbelec tric cell having anode and cathode electrodes, .ani-electrical circuit including a conductive impedance connected between said anode and cathode electrodes, and a condenser of high impedance to desired frequencies produced under control offsaid photoelectric cell and of low impedanceto-undesired frequencies higher than saiddesired frequencies connected to one of said electrodes and in shunt of at least a" portion of said conductivefimpedance. w v i 19. An electro-optical system comprising a photoelec tric .cell having an,. anode and a cathode,'a conductive impedance connected. between said anode, and: ground, a second eonductiveimpedance connected betwe en said cathode and ground," and a' condenser of lowirnpedance to high frequency current connectedv between ls'aid'ariode and'ground. e References'Cited in the fileoit this patent iv i V UNITE S A E rA'rENr Reynolds June 28,1932 1,889,758 Nakke'n L Dec. 6511932 1,973,469 Denis Sept. 11, 1934 2,145,021 Berges' Jan. 24; 1939 2,164,916 Hammond Iuly.-4, 19.39 2,203,882 Shore June 11,1940- 2,219,189 Lundstrom Oct. 22,1940 2,255,245 Y Ferrel Sept. 9, 1941 2 ,360,233 Hussey Oct. 10,1944

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fllherrnistors in Electronic Circuits, Electronic Industries. January .1945, pages 76-80. 

